Visible Light Communication Using OFDM Mostafa Z.Afgani,Harald Haas,Hany Elgala,and Dietmar Knipp
School of Engineering and Science
International University Bremen
28759Bremen,Germany
Email:{m.afgani,h.haas,h.elgala,d.knipp}@iu-bremen.de
Abstract—In this paper wireless communication using white, high brightness LEDs(light emitting diodes)is considered.In particular,the u of OFDM(orthogonal frequency division multiplexing)for intensity modulation is investigated.The high peak-to-average ratio(PAR)in OFDM is usually considered a disadvantage in radio frequency transmission systems due to non-linearities of the power amplifier.It is demonstrated theoretically and by means of an experimental system that the high PAR in OFDM can be exploited constructively in visible light communication to intensity modulate LEDs.It is shown that the theoretical and the experimental results match very cloly,and that it is possible to cover a distance of up to one meter using a single LED.
I.I NTRODUCTION
Recently,there has been incread interest in visible light communication systems.The rearch is motivated by an increasing need of indoor communication systems and the improvements of light emitting diode technologies(LEDs). High brightness LEDs are already ud for veral applications and it is foreen that they will also replace conventional lighting sources in the next decade.Furthermore,the band-width of optical free space communication systems using LED technology is high in comparison to radio frequency bad solutions.This widespread u provides the necessary infras-tructure and hence removes one of the major hurdles faced by new communication schemes;thus making the technology particularly appealing.A typical application scenario might be to additionally u the reading lights in planes for high speed wireless transmission.Moreover,some of the notable advantages of visible light communication over RF(radio frequency)and IR(infra red)bad systems are:
•There are no regulations regarding the u of the visible EM(electromagnetic)spectrum
•Unlike IR communication schemes,there are no health regulations to restrict the transmit power
•Optical communication provides higher curity than RF communication schemes;it is very difficult f
or an intruder to(covertly)pick up the signal from outside the room In an optical communication system,it is possible to modulate the transmitted optical signal in a variety of ways. The pha/frequency,polarization,or the intensity of the optical signal can be modulated.Intensity modulation has the advantage of being particularly easy to implement;the optical output power of the source is simply varied according to the modulating signal.The optical signal thus produced is also easy to detect-the modulating signal is easily recovered from the output of a photodiode.The method is therefore aptly termed direct-detection.The penalty for the simplicity of IM-DD(intensity modulation-direct detection)systems is a reduction in nsitivity,and the vulnerability to noi–as compared to coherent detection heterodyning). OOK(on-off keying),PCM(pul code modulation),and SC-BPSK(sub-carrier binary pha shift keying)are some of the more popular modulation schemes ud in conjunction with LED wireless systems[1].The u of OFDM was first noted in[2].The inherent robustness of OFDM against multipath effects makes it an excellent choice for situations where multiple transmitters are ud simultaneously(to avoid shadowing effects)and a path difference to the receiver exists. One of the major disadvantages of an OFDM bad system is the characteristic high crest factor of the time domain signal. In traditional RF bad systems,this usually necessitates a transmitter with a high dynamic range and hence results in reduced power efficiency.In the propod optical LED system, this disadvantage is turned to an advantage-the time
domain OFDM signal is ud to modulate(IM-DD)the optical source. The signal variations are around an operating point determined by the particular LED in u.It is chon such that the LED operates in the linear region of the current vs.intensity curve. Most of the rearch in the area of visible light communica-tion using OFDM has been theoretical.The goal of the project is to implement an experimental system.This experimental system is primarily aimed at demonstrating a proof-of-concept for the approach of exploiting the high crest-factor in OFDM for LED intensity modulation.Therefore,standard off-the-shelf components were lected.Thereby,it is accepted that the system is clearly not optimid for maximum data rate transmission capabilities.Such optimisation will be carried out at later stages once the feasibly has be shown.
In the next ction,the system design and the optical tup is described.The theoretical and experimental results are described in Section III.Section IV concludes the paper.
II.S YSTEM D ESIGN
The communication chain is implemented using a pair of digital signal processing development kits(Texas Instruments TMS320C6711).The D/A converters of the boards have a precision of16bits and operate at a frequency of8kHz –offering a maximum system bandwidth of4kHz and a sampling i
nterval of125µs.The on-board DSP is capable offloating point operations.With a64-point IFFT(inver
1-4244-0106-2/06/$20.00 ©2006 IEEE
TMS320C6711 DSP TMS320C6711 DSP Fig.1.Block diagram of the OFDM communication chain
0a b c d ···0···d*c*b*a*
Fig.2.Sample IFFT input frame (transpod)
fast Fourier transform)the duration of an OFDM symbol is (64×125µs =)0.008s .With QPSK (quadrature pha shift keying)modulation,the maximum number of bits transferable by each OFDM
symbol is then (64×2=)128.The ideal system is then capable of a maximum raw (assuming no guard or pilot,and all subcarriers are modulated by independent data streams)data rate of 1280.008=
16kbps.However,with the IFFT framing structure chon for the implementation,the maximum achievable data rate (assuming no guard or pilot)is approximately 8kbps.
The following is a description of the various blocks of the chain.Fig.1shows a block diagram of the scheme implemented.A.Transmitter
The transmitter consists of a file processor,QPSK mod-ulator,and an OFDM modulator.The file processor accepts an ASCII data file consisting of the binary data stream.The data read from the file is encoded using the QPSK modulation scheme by means of a lookup table:
00⇒1+j 10⇒-1+j 01⇒1-j
11⇒-1-j
The symbols then undergo a rial to parallel conversion in preparation for the IFFT operation.The parallel data stream is then ud to design the data frame for the IFFT.The first element of the data frame reprents the DC value and hence is chon to be zero.If the number of subcarriers is M ,an
d the IFFT size is N ,then elements 2to M +1of the frame consist of the parallel data and elements N −M +1to N consist of the conjugate complex and mirrored version of the data.If N >2M +1,the intermediate elements are zero.N <2M +1is not feasible and hence the condition N ≥2M +1must be strictly enforced.When N =2M +1,there are no intermediate zero elements.The transpo of a sample frame is shown in Fig.2.For the simple one LED transmitter,
this is necessary to ensure that the output from the IFFT operation is real valued.In favour of simplicity,the resulting reduction in the spectral efficiency by a factor of two is accepted –again,this tup should primarily provide a proof-of-concept.Many white LEDs are in effect a combination of red,green and blue LEDs and hence,it is also possible to u the regular data frame structure (no complex conjugate mirroring)and modulate two of the three LEDs to transmit the real and imaginary parts of the resulting complex OFDM symbols parately.The third LED can be left active but unmodulated to maintain ”whiteness”.Of cour the two LEDs can also be ud with the modified IFFT frame structure to provide full-duplex communication.The receiver would then also have to employ two photo-diodes –each filtered to be nsitive to the wavelength of one of the LEDs.Once the IFFT operation is carried out,the output is procesd to append a 5µs cyclic prefix for protection against multipath effects.The guard length was chon such that any such effects can be safely igno
red.Once the prefix has been added,the parallel data stream is converted back into a rial stream.Before the OFDM symbols are transmitted via the D/A converter,a pilot quence is transmitted.The quence is one OFDM symbol long and consists of two complete sinusoids parated by zeros (Fig.4).The peak-to-peak amplitude of the ”guard sinusoids”is approximately 2.5V and hence they are as strong as the strongest data signal in the time domain waveform.This special structure is needed for successful synchronization at the receiver.The synchronization procedure is detailed in Section II-B.The transmitter then stops processing further data and repeatedly transmits the quence of OFDM symbols and the pilot until a feedback from the receiver is obtained via a wired return channel.Once the transmitter reaches the end of the ASCII data file,it transmits a predefined bit pattern to signal the end of file to the receiver and stops.An OFDM time domain signal depicting the typically high peak-to-average ratio is shown in Fig.3.B.Receiver
The receiver starts by capturing a data stream two frames long.This ensures that the captured data contains at least one
Fig.3.
A Typical Time Domain Signal
Fig.4.The Pilot Symbol
contiguous frame(one pilot symbol and OFDM symbols). Once the symbols have been captured,the receiver signals the transmitter to continue processing the next t of data. The captured symbols are then run through a synchronization detection subroutine.The detector is esntially a running integrator with a window size equal to the length of the silent period(zeros)in the pilot symbol.The integrator scans through the absolute value of the captured data stream one sample at a time and records the index of the position where the minimum value of the integral was obrved.As the integrator only considers absolute values,the integral over a sinusoid is a non-zero number.At this po
int the advantage of the special pilot structure becomes clear–the guard sinusoids provide an effective contrast between themlves and the silent period and hence make it easier for the detector to reach the correct decision.Eventually,the integrator window will come into alignment with the silent portion of a pilot symbol and it is here that the lowest integral will be recorded.Thus the index to the start of the OFDM symbols is determined.Then,using this index,the OFDM symbols are extracted and run through the rial to parallel converter.Once the data is parallelized,the cyclic prefix is removed and the frame is pasd to the FFT operator.The FFT operation reproduces the mirrored frame structure designed in the transmitter.The upper half(elements 2to M+1)of this frame is retained as the valid result.The complex data is then pasd through the QPSK demodulator to recover the binary data.The decision regions defined are:
R(symbol)>0&C(symbol)>0⇒00
R(symbol)<0&C(symbol)>0⇒10
R(symbol)>0&C(symbol)<0⇒01
R(symbol)<0&C(symbol)<0⇒11
The QPSK symbols are also written to an ASCIIfile to facilitate the plotting of receiver constellation diagrams.Once the binary data has been recovered,it is written to an ASCII datafile.Since the transmitted bits are known to the receiver, a bit error rate(BER)calculation is also carried out.At every cycle the receiver checks the received bit quences for a match against the end offile bit pattern;if a match is found,the files are clod and the receiver stops.Otherwi,the program captures the next t of data and repeats the signal processing. It is assumed that the channel is single tap with a real coef-ficient and hence isflat fading.Therefore any pha/amplitude distortion is assumed to be minimal.Conquently,an equal-izer would have very little effect on thefinal result and hence was omitted from the design of this simple transmission chain.
C.The Optical Channel
For the theoretical analysis a suitable channel model is required.The channel model ud is that propod by Barry et al.[3].For the line of sight(LOS)ca with no reflections and assuming that the source and receiver paration squared (R2)is much greater than the receiver area(A R),the channel impul respon can be approximated by a scaled and delayed Dirac delta function
h(t;S,R)≈
n+1
2π
cos n(φ)dΩrect
θ
FOV
δ
t−
R
c
(1) where S is the source and R is the receiver.The simple source is defined as S={r S,ˆn S,n};r S is the position,ˆn S is the orientation,and n is the mode number associated with the directivity of the source and can be calculated from the source half-angle,αH,using(2)[4].The simple detector is define
d as R={r R,ˆn R,A R,F OV};r R is the position,ˆn R is the orientation,A R the receiver area,and F OV thefield of vision.
αH=cos−1(0.5)1n(2) dΩis defined as the solid angle subtended by the receiver’s differential area
dΩ≈cos(θ)
A R
R2
(3)θis the angle betweenˆn R and(r S−r R)
cos(θ)=ˆn R·
(r S−r R)
(4)φis the angle betweenˆn S and(r R−r S)
cos(φ)=ˆn S·
(r R−r S)
R
(5)
Fig.5.Source and Detector Geometry[3] The function rect(x)is defined as:华硕x84h
rect(x)=
1for|x|≤1
0for|x|>1(6)
c is the spee
d of light.Th
e approximation to the impul respon approaches equality as the ratio A R R2approaches zero. The source and detector geometry is best explained by Fig.5. Although an algorithm to derive the channel impul re-spon for multiple reflections is provided in[3],the influence o
f multiple reflections was neglected;as the5µs guard in the OFDM symbols is more than enough to mitigate any multipath effects encountered.
At this point,it is necessary to note that the channel equation
y=r·(x∗h)+˜n(7) relates the transmitted power to the received current.The input signal x is a power signal,the channel transfer function h is dimensionless,and the receiver responsivity factor r reprents the conversion ratio between received optical power and photodiode current at the receiver.Conquently,the received signal y and the additive white Gaussian noi˜n are currents.In optical channels the quality of the transmission is dominated by shot noi;the ambient light striking the detector leads to a steady shot noi that can be considered as a Gaussian noi process.The receiver pre-amplifier noi is also signal independent and Gaussian[5].
D.The Optical Interface
The optical source is a5mm White LED with a luminous intensity of5600mcd.At the normal operational point of the LED,the electrical power output is about72mW.With a 33%efficiency,the optical power output is calculated to be approximately24mW.
The optical detector employs a9.8mm2planar photodiode with a built in infra-red rejectionfilter.The FOV is60◦and the directional nsitivity at0◦is unity.The optical receiver output is biad by+1.5V to ensure that the signal input to the receiver DSP codec falls within the prescribed-0.3V to +3.6V range.Fig.6shows a typical
tup.
Fig.6.The Optical Interface
III.T HEORETICAL&E XPERIMENTAL R ESULTS
In order to compare the experimentalfindings with theo-retical results,a MATLAB simulation chain was designed.In order to simplify the experiments only direct LOS conditions were θ=0◦andφ=0◦.For the simulation chain,time delay in the single path LOS channel is irrelevant since there are no multipath delayed components to consider; therefore,theδ
t−R c
term drops from(1)and leaves an impul respon that is a scalar.As a result,the effect of the channel is fully reprented by the channel DC gain
H(0)=
∞
−∞
介绍中国h(t)dt(8)
困窘For the intensity-in intensity-out channel,this is the fraction of the power from a continuous wave transmitter that reaches the detector[3].
A number of physical parameters such as the source mode number,n,and the responsivity of the photodiode to white light,r,were not available from the datasheets and hence had to be experimentally determined and/or calculated.The datasheet for the white LED specifies a beam half-angle(αH) of20◦.Hence,using(2)the mode number is calculated as45. In order to determine the responsivity of the photodiode to white light from the LED,the current generated in respon to varying receiver-transmitter parations were noted.Since the total optical power transmitted from the optical point source is known,it is possible to calculate the power density(W/m2) over a surface at any arbitrary distance.Thefirst step is to determine the beam solid angleϕ(steradians)given the beam half-angleαH(radians):
ϕ=π·(αH)2(9) Then the surface area subtended byϕat a distance R is ϕR2.The source is left unmodulated and hence has a constant optical power output P T.The power density at an arbitrary
distance R is then P T
ϕR2
.Denoting the received current as I R,the expression for the current density at the receiver in
TABLE I
生活的态度
M EASURED P N[dBW]values for different scenarios
Scenario P N[dBW]
Dark Room-71.6372
Fluorescent Lighting-41.0672
Indoor(Diffud Sunlight)-70.7372
respon to the source is I R A
R .The responsivity can then be
calculated as
r=I R
吐实>中老年奶粉A R
×
ϕR2
P T
.(10)
Although the equation displays an apparent dependency of r on R,this,however,is not the ca–the current,I R,is proportional to1R2and hence counters the effect of the R2 term in the numerator;therefore r is a simple scalar coefficient. From the experimental data,the responsivity of the receiver to white light from the LED was calculated to be0.03A/W. To study the performance of the system three typical sce-narios were considered.Thefirst experiments were carried out in a dark room.The cond t were in a typical office room illuminated byfluorescent lights.Thefinal t of results were obtained in a room illuminated by the diffud sunlight through the windows.In order to have a valid comparison between the simulated results and the experimental results, it was necessa
ry to calculate the approximate noi levels (P N)associated with the three scenarios.This was done by recording the signal at the abnce of the transmitter.As it is assumed that this captured waveform reprents a sample function of an ergodic and zero mean noi process at the receiver,the(linear)noi power in Watts was calculated as
P N≈1
N
·
抵押合同N
i=0
y2i(t)(11)
where N is the number of samples and y i is the received waveform at sample index i.Using the values,the P N in dBW were calculated as shown in Table I.The noi levels for the indoor and darkness scenarios are almost identical since noi in the digital circuitry is the major contributor rather than ambient optical power.
Fig.7shows the distance versus BER plot for the simulated and experimental results.Referring to the P N values asso-ciated with the three scenarios(Table I),it is obrved that the various curves are in agreement.The poor performance of the system underfluorescent lighting can be ascribed to the large amount of impulsive noi that is generated due to the switching nature of thefluorescent technology.As expected,the experimental performance curves are wor than the theoretical curves at the same P N.This is to be expected since the simulation does not account for transmitter noi,loss of synchronization at higher distances,and other implementation loss.
Fig.8shows the signal constellations for the system in the dark and underfluorescent lighting respectively.At a distance
10
10
10
10
10
10
10
100
Distance [mm]
B
E
R
Fig.7.Bit error rate as a function of the distance between the transmitter and the receiver
I−Q Plot: No ambient light (30 cm)
I−Q Plot: Fluorescent lighting (30 cm)
Fig.8.Signal Constellation for QPSK Symbols Received at R=300mm
of300mm,the four regions remain quite distinct for the system in the dark.The illuminated system,however,shows heavy distortion.A spectral analysis of the waveforms ud for calculating the noi power(11)revealed that the latter system has a distorted frequency respon in the1.4kHz to4.00kHz band–affecting more than half of the signal bandwidth.The other systems,however,show no such distortion and poss a flat frequency respon as expected.
It is also clear from the signal constellation that there is a random pha rotation that cannot be due to pha noi alone. It is believed that this rotation is due the lack of precision in synchronizing the received time domain signal.At every cycle, the receiver A/D converter samples slightly ahead or behind the exact time required.From the constellation diagram,the overall pha spread in any one quadrant can be calculated and ud to infer the related(sampling)time delay.In one such experiment,the delay was calculated to be16.67µs.This shows that although the delay is within the sampling interval (125µs),the drift is significant.
IV.C ONCLUSION
The experimental results validate the claim that intensity modulation using OFDM is indeed feasible;the high crest factor that plagues OFDM RF equipment is no longer a disadvantage.The prototype system has managed a distance clo to one meter with an impressive bit error rate of10−3 under the moderate ambient light conditions found indoors. This distance has been achieved with a single LED and without any kind of channel or source coding–it is assumed that an array of similar LEDs with error correction coding would have a much higher intensity and achieve much greater distances with an even lower BER.The system performance curves also display a good match between the theoretical results and the experimental data.It is also clear that the system performance is ultimately dependent on the environment;the interference obrved from thefluorescent lighting system can be easily mitigated by shifting the system bandwidth to a higher frequency.It is possible to further improve the system performance with a faster DSP,better data coding,a higher number of subcarriers,a larger FFT/IFFT,or any combination of the factors.
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有钱任性
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